Pipes humming after replacing toilet fill valves

Two days ago I replaced the fill valve, wall shut-off valve, and connecting hose on a second-floor toilet. The house was built in 1993/4 with all copper pipes. Everything seemed great - no more constant filling of the tank, no more dripping at the shut-off valve. Happy.

Yesterday I did the same thing for a first-floor toilet in the same house. Used same make and model of fill valve. Even managed to get the old compression ring off the copper pipe to put the new one on. No more running in the tank, no more dripping at floor. So far, so good.

Then out of the blue this high-pitched whistling or humming sound began, almost like a tuning fork, and grew louder as it resonated. Flushing the toilet made it go away. I ran the faucets to get any remaining air out of the lines, but this noise keeps coming back intermittently in the pipes, and it's driving me nuts.

An internet search for "humming pipes" often blames this on a toilet fill valve needing to be replaced, but I just did that. Twice. The noise only started after I did the second toilet on the first floor.

Our plumbing contractor friend is one of the many old-timers that like that 400A fluidmaster, and they never miss a chance to say so. Kinda like someone saying, "There's nothing better than a good old fashioned pencil and paper!" But I digress because our friend is kind of missing the big picture about your problem.

There's some teaching on here about water pressure and expansion tanks that might be worth reading, and which I only summarize below, but I think it's the key to your issue, as described below.

You report that the noise goes away when you release pressure in the system by flushing, then returns later. If you didn't have the hum before, then the new stuff you installed is a logical potential source, but maybe not in the way you are thinking. It's not that the new stuff isn't working; it's that it IS working. Here's why:

When water pressure in a home is too high but not high enough to trip, say, the T&P valve on the water heater, the high pressure will find its way out of the system through the least-resistive valves. Often this is an outdoor hose bib or the like that just dribbles a little all day and nobody notices. Very often, it's the toilet fill valve; the pressure just pushes a dribble past it.

And indeed, you mention that the fill valves were running a little. You may be surprised to know that when the fill valve is running, a defective fill valve usually isn't the cause; usually the fill valves are fine, and they are operating properly to refill the toilet because the flapper is failing and letting a small amount of water out of the tank, which the fill valves are duly replenishing. So, often on here, when people say their fill valve needs to be replaced because their toilet is running, we tell them to check the flapper, and, sure enough, when they replace the flapper their "defective" fill valve turns out not to be defective. But that's not the case for you. You DID have a runny fill valve that had nothing to do with the flapper, because when you replaced the valve you solved the problem. If you had a defective flapper like most people do, you would have been telling us that you replaced the fill valve and it DIDN'T solve the problem. But it did. So the fill valves were, in fact, leaking.

The most likely cause of two fill valves simultaneously leaking in a house is higher-than-optimal water pressure pushing around their seals.

You replaced both seals, and they are plainly holding. So...the water is now finding another valve to squeeze out around, and when it does so, wherever it is, it is causing that hum. (This is further shown by the fact that when you replaced only the first valve, no hum. When you replaced the second one, and the toilet fill valves were no longer the weakest seals in the system, the water found somewhere else to start pushing around, leading to the hum.)

One of two possibilities: First, your city pressure is high, and the pressure reducing valve coming into the house is either failing or improperly-set, so you have too much pressure. Second, and probably more likely, you have a "closed" system (either owing to a pressure reducing valve with a check valve, or due to a backflow preventer where the water comes into your home), which is more and more the norm. (Cities are trying to prevent any contamination that occurs of the water in your home system from becoming contamination of the City's system, so they require a check valve or backflow preventer at the water meter.) When your water heater fires up to heat water, the water expands. In the old days, that expansion would be distributed over the entire city water system, because your house water had an open connection to the city water main. In a system with a backflow preventer or check valve, that expansion of water takes place solely within your home system. Because pipes don't expand much, and water doesn't compress well, that expansion takes place in a relatively-small space, leading to a spike in the water pressure (on the hot and cold sides, both). We could be talking as much as 20 psi or more. That's why most new water heater installations have basketball-sized expansion tanks, which are filled with air at a certain pressure and absorb the expanding water from water heating, maintaining a constant pressure. Sometimes, even if you have these, the bladder fails or it needs to be recharged or becomes waterlogged, and its effectiveness fails.

Anyway, to test my theory, get yourself a little cheapo water pressure meter at the hardware store, preferably one with a little telltale that lets you know the max. Hook it up. Take a reading. Flush. When the fill valve shuts off, take a reading. Then, wait until you hear the humming and check the reading. I'm guessing you will find it to be too high, and you can address the cause of the overpressure, whether it's due to your pressure reducing valve near the meter being improperly set (or nonexistent) or due to the absence of an expansion tank (or one that is becoming defective). (BTW, if the pressure after flushing returns immediately to the too-high-level at which the pipes are humming, it's more likely your city water pressure/failed pressure reducing valve. If the pressure stays pretty low, then spikes up when you hear your water heater running, then you know that it's likely thermal expansion.)

Every homeowner should have a pressure gauge. They are cheap and the only way to know for sure exactly what you water pressure is. Many folks believe that high water pressure is a desirable thing, but as wjcandee points out, too much pressure is not a good thing. Over 80 psi is too much, and actually somewhere around 50 to 60 psi is plenty. Understand that city water pressure can vary a great deal over a 24 hour period. Can get very high at night when there is little demand and seem quite OK during the day when there is normal usage. A PRV will prevent the peaks, but does require an expansion tank to absorb the expansion in your pipes caused by heated water. Both the PRV and expansion tank are mechanical devices and will occasionally fail and need replacing. That's one reason you need a pressure gauge. The usual sign of the lack of a functioning expansion tank is the T/P on the water heater opening every time the heater comes on.

Thank you very much to everyone. It's wonderful to be pointed in a direction, where previously I had none. Before anyone else had replied, I did follow john's advice, and swapped out the fill valves (again) for new Fluidmaster PerforMAX models. If nothing else, they do seem sturdier, and I'm pleased with them, though that did not solve anything.

wjcandee, I think you have hit the nail on the head. Everything you describe sounds like what I've been experiencing, and the reasoning behind why it would start only after the second leaking valve was replaced makes perfect sense. Today I did some further experimenting, and discovered I can make the sound vanish by opening any faucet in the house too, which only reinforces the idea that it's pressure seeking a release somewhere. At least I now know my work was sound, and just happened to uncover a root issue. (Btw, I replaced my first flapper when I was eight years old, and knew that wasn't the culprit here. I admire the way you patiently explained everything to walk me through each step and follow the train of thought. It's helpful.)

Looks like my next step will be to get a pressure gauge. To where do you recommend I hook it up? That part isn't clear to me.

Gary, thanks for the additional info about PRVs. hj, no outdoor faucets left on here, though tonight I'll be leaving a few bathroom faucets dripping to relieve the pressure so I can sleep through the night! Today was a holiday, but tomorrow it's back to work, and I can't go through last night again.

You can buy a pressure gauge with a hose end - it attaches like a hose. Now, where in the house you may have one of those would depend: outside hose bib (maybe not a good idea this time of year!), the washing machine, a utility sink, the drain on the water heater (careful, it will get hot!), or with adapters, after removing an aerator on a faucet, on any one of those.